Automatic apparatus for spraying plate-shaped bodies

ABSTRACT

Automatic apparatus for spraying plates including a roller conveyor for conveying plates along a path, at least one spray gun for spraying the plates during movement, driving mechanism for reciprocating the spray gun across the conveyor path between opposite sides of the plates and an electronic sensing apparatus for controlling starting and stopping of the paint supply to the spray gun. The sensing apparatus includes two electromagnetically operable width measuring sensors at the inlet of the conveyor, each sensor having a movable carriage, two motors for respectfully driving the carriage, and a control means for starting the motors to move the sensors from a first position in the vicinity of the longitudinal centerline of the conveyor path in opposite directions across said path towards the opposite edges of the plate to outer positions corresponding to the opposite side edges of the plate. Additionally included is an electric bridge circuit including a potentiometer having a displaceable tap whose axis is mechanically coupled to the motors of the carriages; said tapping arranged such that in the outer position of a sensor carriage, the tap determines the position of one side edge of the plate in relation to the longitudinal axis of the conveyor path.

United States Patent [72] Inventor Egon Verner Christensen Copenhagen, Denmark [21] Appl. No. 694,001 [22] Filed Dec. 27, 1967 [45] Patented Mar. 2, 1971 [73] Assignee H. Nielsen & Son Maskinfabrik A/S Herlev, Denmark [54] AUTOMATIC APPARATUS FOR SPRAYING PLATE-SHAPED BODIES 7 Claims, 3 Drawing Figs.

[52] US. Cl. 118/2, 118/8, 118/316, 118/323 [5 I] Int. Cl B05c ll/00 [50] Field of Search 118/2, 8, 323, 316

[56] References Cited UNITED STATES PATENTS 3,029,774 4/1962 Namenyi-Katz 1 18/2 3,255,037 6/1966 Knight et al. 118/2X r w Mi 'm? my l 9 A 3,566,&28

Primary Examiner-John P. McIntosh A ttorney- Beveridge & DeGrandi ABSTRACT: Automatic apparatus for spraying plates including a roller conveyor for conveying plates along a path, at least one spray gun for spraying the plates during movement, driving mechanism for reciprocating the spray gun across the conveyor path between opposite sides of the plates and an electronic sensing apparatus for controlling starting and stopping of the paint supply to the spray gun. The sensing apparatus includes two electromagnetically operable width measuring sensors at the inlet of the conveyor, each sensor having a movable carriage, two motors for respectfully driving the carriage, and a control means for starting the motors to move the sensors from a first position in the vicinity of the longitudinal centerline of the conveyor path in opposite directions across said path towards the opposite edges of the plate to outer positions corresponding to the opposite side edges of the plate. Additionally included is an electric bridge circuit including a potentiometer having a dispiaceable tap whose axis is mechanically coupled to the motors of the carriages; said tapping arranged such that in the outer position of a sensor carriage, the tap determines the position of one side edge of the plate in relation to the longitudinal axis of the conveyor path.

PATENTED MAR 2197! SHEET 1 OF 2 INVL'INTOR.

ER CHRISTENSEN EGON VERN AUTQMATIC APE ARATUS FOR SPRAYING kLATlE- SHAPED BODHES BACKGROUND OF THE INVENTION l. Field of the invention Automatic painting plant for coating plate-shaped bodies and having at least one spray gun, which is moved at right angles to the path along which the plate-shaped body is advanced by a substantially plane roller conveyor and having means for opening and closing of the spray guns used, said means being activated and deactivated when the spray gun in its path reaches the one edge and leaving the other edge of the plate-shaped body respectively.

2. Description of the Prior Art Means for detecting the side edges of the plate-shaped body are sensitive to sound or light and activated by an ultrasonic sound generator, or by a source of light, said means being disposed. adjacent to the spray guns and the edges of the body, opening and closing respectively for the feed of paint material to the spray gun. Said means and the corresponding generator or source are therefore liable to be fouled with paint. This involves disadvantages in the form of a frequent cleaning of the detecting means, generators or sources, waste of paint material and risk of failure in the control of the feed of paint material to the guns.

SUMMARY OF THE INVENTION The plant has electronic sensors provided at the entrance edge of the roller conveyor adapted to act upon control means for starting driving motors for at least two measuring sensors, being arranged moveable in opposed direction at right angles to the longitudinal center plane of the roller path, and being stopped in positions corresponding to one and the other side edges of the plate-shaped body, said position being indicated by the tap of a potentiometer inserted in an electric bridge circuit and said potentiometer constitutes the two arms in said bridge circuit and the potential of said tap is sensed by the displaceable tap of another potentiometer which constitutes the two other arms of the bridge circuit and last-mentioned tap being mechanically coupled to the spray gun. When balance is obtained in the bridge circuit the feed of paint to the spray gun is started and stopped respectively.

The object of the invention is to devise an automatic painting plant for coating plate-shaped bodies or objects and having at least one spray gun which is moved at right angles to the path along which the plate-shaped body orobject is advanced by a substantially plane roller conveyor.

Painting plants of the aforesaid kind are known, and for controlling the opening and closing of the spray gun use is made, say, of detector means sensitive to sound or light and activated by a sound generator, which, for example, may produce ultrasonic vibrations, or by a source of light, respectively, the said detector means being so disposed in relation to the generator or the source of light that they are covered by the plate-shaped body during the movement of the spray gun above or beneath the body, but react when the gun is displaced into position inside or outside the edge of the object, admitting and cutting off, respectively, the feed of paint to the spray gun.

These plants have, however, the drawback that the detectors are arranged inside the painting cabin of the plant directly adjacent to the spray guns, and they are therefore liable to be fouled with paint owing to their location near the spray guns. This involves disadvantages in the form of a frequent cleaning of the detectors and requires an unsuitable, complicated mechanical design for carrying the said detectors. Furthermore, the arrangement of the detectors in connection with the two movable spray guns involves that flexible cables have to be led to these detectors, which entails the risk of wear and damage with consequent breakdowns in operation and difficult replacement of the movable cables inside the painting cabin.

These drawbacks are relieved by an automatic painting plant according to the invention wherein the plant has electronic sensors provided at the entrance edge of the roller conveyor and adapted to control starting and stopping means for the feed of the paint to the spray gun in a part of its painting stroke above and beneath the surfaces of the plate-shaped object, the said part of the painting stroke being determined by the transverse and longitudinal dimensions of the object.

As a result, the electronic sensors maybe placed outside the direction of the spray ejected by the guns and outside the painting cabin of the plant, by which they are protected from being fouled by paint in any way whatever, so that the plant operates with great reliability and precision. As a further result the spray gun is moved uniformly in the entire maximum width of the painting zone, the stretches of acceleration and retardation lying outside the fixed painting zone which is determined by the width of the object. Thus, the spray gun is only active in such part of its path as is located between the side edges of the plate-shaped object. There is therefore no waste of paint along such part of the path of the spray gun as is located outside the side edges of the plate-shaped object, just as the roller conveyor is not liable to be fouled by paint.

in one embodiment of the plant according to the invention two electromagnetically or electrostatically operable width measuring sensors are provided at the entrance edge of the roller conveyor, each on its separate, displaceable sensor carriage, and adapted to be activated when the leading edge of the plate-shaped object is at a predetermined location in the roller path and adapted to act upon control means for starting the driving motors of the sensor carriages so as to move these from their inactive position in the vicinity of and on opposed sides of the longitudinal center plane of the roller path and in opposed directions of each other at right angles to the said center plane, and for stopping the sensor carriages when each width measuring sensor has reached its separate outer position which corresponds to one and the other side edges, respectively, of the plate-shaped object advanced along the roller path.

As a result, the width measuring sensors detect in their outer position the width of the object to be spray-painted.

In a preferred embodiment of the plant according to the invention there is mechanically coupled to each of the driving motors of the sensor carriages a displaceable tap of a potentiometer inserted in an electric bridge circuit, the position of the said tap constituting in the outer position of the sensor carriage a measurement of the dimension of the plate-shaped object in the transverse direction of the roller conveyor.

As a result, the potential of the tap may be utilized in the main circuit in which the potentiometer constitutes the two adjacent arms of the bridge and here may be sensed by the displaceable tap of the other potentiometer, which tap is mechanically coupled to the spray gun and therefore follows the movement of same during each painting stroke; the said other potentiometer constitutes the two other adjacent arms of the bridge so that the bridge is in balance when the two taps adopt the same angular position in relation to the outer terminals of the respective potentiometers, the said balance position being utilizable for starting and stopping the feed of paint to the spray guns by means of an electronic relay inserted in the diagonal of the bridge.

in a third embodiment of the plant according to the invention a stationary electronic sensor, likewise provided at the entrance edge of the roller conveyor, is adapted to start a first impulse counter which is adapted to count at predetermined number of voltage impulses from an impulse transmitter working synchronously with the driving shaft of the roller conveyor and, upon completion of the said counting, to start the first painting stroke of the spray gun and another impulse counter adapted to start each of the subsequent periodic painting strokes of the spray gun, the said other impulse counter being electrically connected to another impulse transmitter, likewise working synchronously with the driving shaft of the roller conveyor.

As a result, the time for starting the first painting stroke of the spray gun may be determined in relation to the advancement of the plate-shaped object in the roller conveyor. As a further result, the other impulse transmitter may determine the number of painting strokes per unit of length by which the plate-shaped object is advanced in the roller conveyor.

In a fourth embodiment of the plant according to the invention the other impulse transmitter is coupled to the driving shaft via a variable gear, as a result of which the number of painting strokes per unit of length of the said advancement of the object in the longitudinal direction of the roller conveyor may be regulated for variation of the amount of paint fed per unit of length of the said advancement so that the individual painting strokes overlap each other by predetermined amounts of overlappings.

In a fifth embodiment of the plant according to the invention each impulse transmitter consists of a vane wheel mounted on the driving shaft of the roller conveyor or on another shaft moved synchronously with said shaft, the said vane wheel cooperating mechanically or electrically with an impulse member at each passage of a vane.

As a result, a simple and reliable timed coupling of the rate of movement of the roller conveyor with the painting stroke of the spray gun is obtained, independently of the speed at which the roller conveyor is advancing the object at any given moment.

In a sixth embodiment of the plant according to the invention the vane wheel is made of magnetizable material and its vanes are passed through an air gap in a magnetic circuit to generate induced voltage impulses in a coil surrounding a part of the magnetic circuit, by which a very simple and reliable generation of impulses is obtained.

In a seventh embodiment of the plant according to the invention the local rate of movement of the spray gun is deter mined by a parallel guide system consisting of a lever system which at its one end is coupled to a driving mechanism and at its other end is coupled to the spray gun which rests in a rectilinear guide rail provided transversely of the roller path.

As a result, s substantially uniform movement is imparted to the spray gun in the major part of its path.

In an eighth embodiment of the plant according to the invention a tap of an electric potentiometer is mechanically coupled to the lever system in such manner that the tap is in identical positions corresponding to the position at any given time of the spray gun in its path of movement.

As a result, the potentiometer, which is mechanically coupled to the lever system, may in an electric bridge circuit be caused to cooperate with the potentiometers of the driving motors for the sensor carriages in such manner that the feed of paint to the spray gun is started and stopped when there is coincidence between the angular positions of the potentiometers.

In a ninth embodiment of the plant according to the invention a set of spray guns is provided both above and beneath the path of the plate-shaped object, as a result of which a coating may be applied to the upper surface of the object along a part of its path, and a coating may be applied to the underside of the object along the same or another part of its path.

BRIEF DESCRIPTION OF THE DRAWING The invention will now be further described with reference to the drawing, in which:

FIG. 1 is a diagrammatical representation, showing in perspective the essential parts of a painting plant according to the invention;

FIG. 2 shows a parallel guide system for the spray guns of the painting plant; and

FIG. 3 is a diagrammatical representation of the mode of chronological operation of the painting plant.

FIG. 1 shows a plane roller conveyor 1 consisting of a number of rollers 2,3 driven by a driving shaft 4 from an electromotor (not shown) which is also driving all the rollers 2,3 by means ofa chain drive 5. On the roller conveyor 1 is moved a plate-shaped object 6 to which at least one coating is to be applied by means of spray guns 7 and 8 mounted in pairs, one set being disposed above the object 6 and the other set 8 beneath the object 6, but the latter set is omitted in FIG. 1 for the sake of clarity. The spray guns 7 and 8 are displaceably mounted, each on its separate bridge 9 which is located at right angles to the direction of advancement of the plateshaped object as indicated by an arrow 10. The spray guns are displaced along the bridges by means of parallel guides 11 and 11: each of which includes a first pair of levers 11a pivoted at S and T at one of their ends with respect to the spray gun support and at their opposite ends at Q and R with respect to a second pair of levers 11b. One of the end portions of levers 11b is pivotally connected at V to the actuating plunger X of a hydraulic motor 12 to be actuated thereby for purposes of pivoting levers 11b relative to bridge 9 about pivot points 13 and 14; it being understood that the parallel guides 11 and 11: are pivotally supported in the bridge 9 at points 13 and 14 and 13: and 14:, respectively. Hydraulic cylinder 12 is suitably pivoted to a stationary point in the frame of the plant at point P. The parallel guides ensure a uniform movement of the spray guns from an outer position 15, indicated by a solid line in FIG. 3, to another outer position 16, indicated by a dotted line in FIG. 3. When hydraulic cylinder 12 is actuated, for example, to extend operating plunger X for pivoting lever 11b counterclockwise (as shown in FIG. 2) about pivot points 13 and 14 on bridge 9, this will also actuate levers 11a about pivot points Q and R in a clockwise direction into the positions shown in dotted lines in the center portion of FIG. 3. When hydraulic cylinder 12 is actuated to retract operating plunger X, levers 11a and 11b are returned to the positions shown in FIG. 2. On each of the parallel guides is provided an electric potentiometer 17 and 18, respectively, connected to a direct current source, the tap of the said potentiometer being mechanically coupled to the lever system in such manner that the tap of the potentiometer adopts a position indicating the position of the corresponding spray gun in the path of the said gun. Potentiometers 17 and 18 are mounted in any suitable manner with respect to the parallel guides with the tap of the potentiometer being connected to the operating plunger X of hydraulic cylinder 12 at point U shown in FIG. 2. Thus, movement of operating plunger X of hydraulic cylinder 12 provides a corresponding movement in the tap of potentiometer 17 or 18.

To the driving shaft 4 are mechanically coupled two impulse transmitters S1 and S2 shown in FIG. 1. The said impulse transmitters consist each of a vane wheel 19 cooperating strongly mechanically or electrically with an impulse member whenever a vane is passing. The vane wheel 19 may, for example, be made of magnetizable material and its vanes be passed through an air gap in a magnetic circuit 20, by which voltage impulses are generated in a coil 21 surrounding a part of the magnetic circuit 20. The said voltage impulses, the number of which per second is determined by the speed of the driving shaft and by the number of vanes, is tapped via terminals 22 and 23 on the coil 21. The two impulse transmitters may thus generate two different series of voltage impulses which are used to determine the time intervals for activation of the different components of the painting plant, such as start and stop of the painting stroke of the spray guns, and the number of painting strokes per unit of length of the path along which the plate-shaped object is moving. Impulse transmitter S2 may be coupled to driving shaft 4 via a variable gear G as shown in FIG. 1 as a result of which the number of painting strokes per unit of length of advancement of the object in the longitudinal direction of the roller conveyor, may be regulated for variation of the amount of paint fed per unit of length of said advancement so that the individual painting strokes overlap each other by predetermined amounts. The number of voltage impulses is counted by means of impulse counters I, II, III, IV and V electrically connected to the impulse transmitters Sland S2 and consisting, for example, of electrical step-by-step selectors which are displaced by one step per voltage impulse and the selector contacts of which are provided with intermediate connections to determine the number of impulses between eachactivation of the aforesaid components of the plant as will be described in the following.

At the entrance edge of the roller conveyor 1 and outside the cabin wall K of the plant are provided three electronic sensors 24, 25 and 26 which, for example, may be of electromagnetic type and be connected to an alternating current mains and an alternating current bridge in such manner that the bridge is normally in balance, while a relay, mounted in one diagonal of the bridge, is in inactive position when there is no magnetizable object such as an edge or a surface of the plateshaped object within the magnetic field of the sensor, but is brought into unbalanced state, in which the relay is activated, when one of the edges of the body to be painted is brought into magnetic field of the sensor.

The first sensor 24 is stationary in the longitudinal center plane of the roller conveyor and is adapted to be activated when the object is passing the sensor. The two other sensors 25 and 26, termed width-measuring sensors, are disposed symmetrically about the said center plane and are displaceable on a rail 37, each carried by its separate sensor carriage 27 and 23, respectively, which is mechanically coupled, for example by a gearwheel drive or a chain drive, to its separate electromotor 29 and 30, respectively, which, when the said electromotoris fed'with current, imparts a motionto the sensor carriages Z7 and 28 by which they are moved from their neutral position in the vicinity of the longitudinal center plane of the roller conveyor in a direction at right angles away from the said plane. The two width-measuring sensors 25 and 26 are each coupled to their separate electric circuits, for example to an alternating current bridge, which is in balance, and a relay mounted in one diagonal of the bridge is in its inactive position when there is no magnetizable body such as a-sideedge of the plate-shaped body within the magnetic field of the sensor, whereas the said alternating bridge is brought into unbalanced state in which the relay is activated when the surface of the object to be painted or its side edges are within the magnetic field of the sensor. Onthe shafts 31 and 32 of the electromotors 29 and 30 are mounted two electric potentiometers 33 and 34 which are connected to a direct current source and the taps of which are connected via electronic no-volt relays 33a and 3441, respectively, to the taps of the potentiometers 17 and 18 on the parallel guides 11 and 11:, for the purpose which will be further described in the following.

Referring to FIG. '3, potentiometer 17 is connected to potentiometers 33 and 34 associated with motors 29 and 30 which drive movable sensors 25 and 26. The connection between potentiometers l7 and potentiometers 33 and 34 is effected through relays 33a and 34a. The other potentiometer 18 associated with the spray gun on the opposite side of the plate being sprayed, is connected to the potentiometers associated with the other pair of motors for driving the other pair of movable sensors on the opposite side of the plate. Relay counters I, ll, III, IV, and V are electrically connected in any suitable manner to impulse transmitters Sland S2 The mode of operation of the automaticpainting plant is as follows:

1. Painting of the Upper Side of the Object When a plate-shaped object such. as the front edge of an iron plate or sheet is introduced into the roller path at the entrance edge of the conveyor as indicated by a line A in FIG. 3, the two width-measuring sensors 25 and 26 activate via contactors (not shown) both of the electromotors 29 and 30, by which the sensor carriages 27 and 28 by means of the gear-wheel drives or chain drives are moved away from their inactive position until the two width-measuring sensors 25 and 26 reach the side edges of the plate, in which position the two relays in the alternating current bridges each cut out and stop the movement of the sensor carriages. The amount of movement is simultaneously registered by the taps of the two potentiometers 33 and 34 being turned, by which an electric indication of the width of the plate-shaped object isobtained, the said indication being, as will be explained later on, used for determining the length of the active painting stroke of the spray guns.

Simultaneously with the front edge of the plate-shaped object being introduced into the roller path the stationary sensor 24 is activated, and the relay corresponding to the alternating current bridge of the said sensor drives and cuts in the impulse counter I which the impulse transmitter Slfeeds with voltage impulses of a fixed number of cycles in relation to the speed of the driving shaft, and when the counter I has counted a predetermined number of impulses, the front edge of the plate-shaped object 6 has reached a position indicated in FIG. 3 by a line B. The predetermined painting strokes of one spray gun 7 are now started via the one selector contact of the impulse counter I, and at the same time the other impulse counter ll, which is connected to the other impulse transmitter S2 starts and determines the number of painting strokes per unit of length of the path alongwhich the plate is advanced. The impulse transmitter S2 may be adapted to transmit a regulable number of impulses per unit of length, by which the overlapping zones of the painting stroke may be regulated based upon the knowledge of thecoating range of the spray guns. The potentiometer 17 which is mounted on the parallel guide 11 and the tap of which is moved timed to the painting strokes of the spray guns 7 is coupled together via the no-volt relays 33a and 34a with the potentiometers 33 and 34 in such manner that in combination they constitute a bridge coupling the no-volt relays 33a and 34a of which are adapted to admit and cutoff the feed of paint when the 'position of the tap of the potentiometer 17 corresponds to the position of the potentiometers 33 and 34, that is, when the spray guns cross the I plate side edges indicated in F l0. 3 by the dotted lines 35 and 36. As a result, the spray guns 7 are only ejecting paint when moving across the upper side of the plate, but not when moving across its side edges 35 and 36.

It will be appreciated that the outer positions 15 and 16 of the painting stroke may similarly be determined and regulated by other, for example manually adjustable potentiometers corresponding to the potentiometers 33 and 34 and cooperating in corresponding manner with the potentiometer 17 of the parallel guide 11.

When the plate 1 has reached a line C, the impulse counter l stops, but remains in its final position until the trailing edge of the plate has deactivated the sensor 24. However, the impulse counter ll continues counting the impulses of painting strokes, and when the trailing edge of the plate crosses the line A, the sensor 24 is deactivated, by which the impulse counter l is promptly reset in its neutral position, but is restarted immediately to count a new train of impulses the number of which registers the movement of the trailing edge of the plate, while the impulse counter ll continues controlling the number of painting strokes. When the trailing edge of the plate reaches the line C, the impulse counter l and the impulse counter ll are reset in their neutral positions, and the plant is ready to receive a fresh plate. However, any painting strokes commenced are completed, and after the finishing painting stroke the width-measuring sensors 25 and 26 are moved into their neutral positions.

2. Painting of the Underside of the Object When the leading edge of the plate 6 reaches the line A and the sensor 24 is activated, also a third impulse counter ill will be started, and when the leading edge of the plate reaches a line D, the said third impulse counter-starts a set of other motors (not shown) corresponding to the motors 29 and 3t and consequently also provided each with its potentiometer corresponding to the potentiometers 33 and 34. These other motors stop when their potentiometers have attained the same position as the potentiometers 33 and 34, by which the width measurement of the plate is registered for the use of the other set of spray guns 8 mounted beneath the plate. In similar manner the painting strokes of the other set of spray guns 8 are started by the impulse counter ll when the plate has reached a line E, and at the same time a fourth impulse counter IV is started which determines the number of painting strokes performed by the said spray gun set transversely of the longitudinal direction of the roller path, similarly as by the control of the first set of spray guns, the potentiometer 18, which is mounted on the parallel guide 11:, cooperating with the potentiometers corresponding to the potentiometers 33 and 3 which control the feed of paint in accordance with the side edges 35 and 36 of the plate. When the leading edge of the plate reaches a line F, the third impulse counter lll stops, whereas the impulse counter lV continues counting impulses transmitted by the transmitter S2 When the trailing edge of the plate passes the sensor 24 and deactivates same, the counter lll returns to its neutral position and is thus ready to receive a fresh plate, while a fifth impulse counter V starts to count impulses until the trailing edge of the plate crosses a line F, after which both the counter IV and the counter V return to their neutral positions and so are ready to receive a fresh plate. However, any painting strokes commenced are completed, and upon their completion the potentiometers are returned to their neutral position by the driving motors of the sensor carriages.

3. Painting of Objects with Oblique Side Edges If a plate 6 having an oblique side edge 35 or 36 or two oblique side edges 35 and 36 is to be passed through the plant, the plate has to be advanced with its minimum width leading, since the sensor carriages 27 and 28 are only adapted to register increasing widths. If such a plate .is passed through the plant with its maximum width leading, the plate will be painted as though it were of the said width throughout its length, by which there would be a waste of paint.

lclaim:

1. An automatic painting plant for applying a uniform coating to plate-shaped objects, comprising, a roller conveyor for moving the plate-shaped objects along a first path, at least one spray gun for spraying one surface of the plate-shaped object during movement along said path, driving means for mechanically reciprocating by strokes of constant length the spray gun across the conveyor path between the opposite side edges of the object, electronic sensor means for controlling starting and stopping of means for feeding paint to the spray gun in a regulatable central part of its painting stroke, said part of the painting stroke being determined by the transverse and longitudinal dimensions of the object, said sensor means including two eiectromagnetically operable width-measuring sensors provided at the inlet end of the roller conveyor, each sensor having a movable carriage, two motors for respectively driving each sensor carriage, control means for starting said motors to move the sensors from a first position in the vicinity of and on opposite sides of the longitudinal centerline of said conveyor path in opposite directions across said path towards the opposite side edges of the object to a second position in which the sensors have reached outer positions corresponding to the opposite side edges of the plate-shaped object, said control means including an electric bridge circuit including a potentiometer having a displaceable tap whose axis is mechanically coupled to said motors of the sensor carriages, said tap being arranged such that in the outer position of a sensor carriage the tap determines the position of one side edge of the object in relation to the longitudinal axis of the conveyor path.

2. A plant defined in 1 wherein said roller conveyor has a drive shaft and wherein there is further included first and second impulse counters and associated first and second impulse transmitters operable synchronously with said drive shaft of the roller conveyor, and a stationary sensor provided at the inlet of said roller conveyor for starting the first impulse counter to count a predetermined number of voltage impulses supplied from the associated impulse transmitter and upon completion of said counting to start the driving means for a first painting stroke of the spray gun and the second impulse counter to start each of a predetermined number of subsequent painting strokes of the spray gun.

3. A plant as claimed in claim 2, the other impulse transmitter is cou led to the drivin shaft via a variable gear.

4. A plan as claimed in c mm 3, each impulse transmitter consists of a vane wheel mounted on the driving shaft of the roller conveyor or on another shaft moved synchronously with said shaft, the said vane wheel cooperating mechanically or electrically with an impulse member at each passage of a vane.

5. A plant as claimed in claim 4, in the vane wheel is made of magnetizable material and its vanes are passed through an airgap in a magnetic circuit to generate induced voltage impulses in a coil surrounding a part of the magnetic circuit.

6. A plant as claimed in claim 5, in the local rate of movement of the spray gun is determined by a parallel guide system consisting of a lever system which at its one end is coupled to a driving mechanism and at its other end is coupled to the spray gun which rests in a rectilinear guide rail provided transversely of the roller path.

7. A plant as claimed in claim 6, in a tap of an electric potentiometer is mechanically coupled to the lever system in such manner that the tap adopts identical positions corresponding to the position at any given time of the spray gun in its path of movement. 

1. An automatic painting plant for applying a uniform coating to plate-shaped objects, comprising, a roller conveyor for moving the plate-shaped objects along a first path, at least one spray gun for spraying one surface of the plate-shaped object during movement along said path, driving means for mechanically reciprocating by strokes of constant length the spray gun across the conveyor path between the opposite side edges of the object, electronic sensor means for controlling starting and stopping of means for feeding paint to the spray gun in a regulatable central part of its painting stroke, said part of the painting stroke being determined by the transverse and longitudinal dimensions of the object, said sensor means including two electromagnetically operable width-measuring sensors provided at the inlet end of the roller conveyor, each sensor having a movable carriage, two motors for respectively driving each sensor carriage, control means for starting said motors to move the sensors from a first position in the vicinity of and on opposite sides of the longitudinal centerline of said conveyor path in opposite directions across said path towards the opposite side edges of the object to a second position in which the sensors have reached outer positions corresponding to the opposite side edges of the plate-shaped object, said control means including an electric bridge circuit including a potentiometer having a displaceable tap whose axis is mechanically coupled to said motors of the sensor carriages, said tap being arranged such that in the outer position of a sensor carriage the tap determines the position of one side edge of the object in relation to the longitudinal axis of the conveyor path.
 2. A plant defined in 1 wherein said roller conveyor has a drive shaft and wherein there is further included first and second impulse counters and associated first and second impulse transmitters operable synchronously with said drive shaft of the roller conveyor, and a stationary sensor provided at the inlet of said roller conveyor for starting the first impulse counter to count a predetermined number of voltage impulses supplied from the associated impulse transmitter and upon completion of said counting to start the driving means for a first painting stroke of the spray gun and the second impulse counter to start each of a predetermined number of subsequent painting strokes of the spray gun.
 3. A plant as claimed in claim 2, the other impulse transmitter is coupled to the driving shaft via a variable gear.
 4. A plant as claimed in claim 3, each impulse transmitter consists of a vane wheel mounted on the driving shaft of the roller conveyor or on another shaft moved synchronously with said shaft, the said vane wheel cooperating mechanically or electrically with an impulse member at each passage of a vane.
 5. A plant as claimed in claim 4, in the vane wheel is made of magnetizable material and its vanes are passed through an airgap in a magnetic circuit to generate induced voltage impulses in a coil surrounding a part oF the magnetic circuit.
 6. A plant as claimed in claim 5, in the local rate of movement of the spray gun is determined by a parallel guide system consisting of a lever system which at its one end is coupled to a driving mechanism and at its other end is coupled to the spray gun which rests in a rectilinear guide rail provided transversely of the roller path.
 7. A plant as claimed in claim 6, in a tap of an electric potentiometer is mechanically coupled to the lever system in such manner that the tap adopts identical positions corresponding to the position at any given time of the spray gun in its path of movement. 